The invention relates to bi-polar transducers and more particularly, to a transducer for logging boreholes.
Open boreholes are logged with acoustic pulses to determine the velocities of compressional and shear waves traveling through the earth formations surrounding the borehole. By timing the travel of acoustic waves between the transmitters and receivers of a logging tool, the nature of these surrounding formations is determined. U.S. Pat. No. 4,516,228--Zemenek describes a logging tool for detecting both compressional and shear waves.
Monopole transducers typically generate a compressional wave by generating a pressure pulse on one side of the transducer which radiates outwardly from the transducer. Bender transducers typically are monopole transducers which generate compressional waves U.S. Pat. Nos. 3,363,118--Sims and 3,380,019--Sims disclose bender transducers for use in open water. These transducers include a disk and a piezoelectric material which flexes the disk in a bender action. The disk is fixed around its edge, as by a support ring in the Sims '118 patent so that when a voltage is applied to the piezoelectric material, the center portion of the disk flexes. Alternatively, these transducers are used as receivers which convert acoustic energy to an electrical signal.
U.S. Pat. No. 4,383,308--Caldwell discloses the use of a bender transducer in a borehole logging tool. The operating environments of a borehole and open water are quite different, as are the respective transmission media and associated instrumentation. A transducer used in a logging tool must be capable of operating under high temperatures, typically 300.degree.-350.degree. F.
Another known type of acoustic transducer is referred to as a "bender bar" transducer, such as the one made by Honeywell, Inc. This comprises a stack of flat piezoelectric elements which are supported at one end. When a voltage is applied, the other end of the stack moves to produce a monopole pulse of energy. One way to use bender bar transducers in a logging tool would be to provide a circular array of such devices, with each bender bar stack producing energy which radiates outwardly from the tool. However, the constraints of a logging tool do not provide enough room for such an array of sources. The industry standard logging tool is 35/8" in diameter which severely constrains the size of the transducer.
It is an object of the present invention to provide a bender transducer which will fit into an industry standard logging tool, and which will withstand the severe conditions of borehole logging.
It is another object of the present invention to provide a bender transducer which will radiate a dipole wave with positive pressure pulses on one side of the transducer, and negative pressure pulses on the other side of the transducer.